Liquid crystal display

ABSTRACT

A liquid crystal display comprises two parallel spaced substrates and a liquid crystal layer with negative dielectric anisotropy interposed between the substrates. The ratio d/p, the cell gap d between the substrates to the pitch p of the liquid crystal layer, is equal to or less than 0.3, and the retardation value Δn*d may be in the range of 0.25-0.4. In absence of electric field, the liquid crystal molecules are arranged vertically to the substrates, and when the sufficient electric field is applied, the liquid crystal molecules are parallel to the substrates and twisted by 90° from one substrate to the other.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/765,605 filed on Apr. 22, 2010, which is a continuation of U.S.application Ser. No. 11/204,041 filed on Aug. 16, 2005, issued as U.S.Pat. No. 7,929,088, which is a divisional application of U.S. Ser.No.10/655,516 filed on Sep. 4, 2003, issued as U.S. Pat. No. 6,943,858,which is a divisional application of U.S. Ser. No. 10/114,718 filed onApr. 1, 2002, issued as U.S. Pat. No. 6,646,701, which is a divisionalapplication of application Ser. No. 09/087,628, filed on May 29, 1998,now abandoned, which claims priority to and the benefit of Korean PatentApplication No. 1997-21709, filed on May 29, 1997 and Korean PatentApplication No. 1997-28480, filed on Jun. 27, 1997, the disclosures ofwhich are incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to liquid crystal displays using verticalalignment and compensation films.

(b) Description of the Related Art

A liquid crystal display has two substrates opposite each other and aliquid crystal layer interposed between the substrates. If the electricfield is applied to the liquid crystal layer, the liquid crystalmolecules changes their orientations to control the transmittance of theincident light.

A twisted nematic (TN) liquid crystal display includes a couple oftransparent substrates having transparent electrodes thereon, a liquidcrystal layer interposed between the substrates, and a couple ofpolarizers which are attached to the outer surfaces of the substrates.In off state, i.e., in absence of the electric field, the molecular axesof the liquid crystal molecules are aligned parallel to the substratesand twisted spirally by a constant pitch from one substrate to the othersubstrate, and the director of the liquid crystal layer variescontinuously.

However, the contrast ratio of the conventional TN mode liquid crystaldisplay, especially in normally black mode, may not be so high becausethe incident light is not fully blocked in absence of the electricfield.

To solve this problem, a vertically aligned twisted nematic (VATN) modeliquid crystal display is proposed in the U.S. Pat. No. 5,477,358,“CHIRAL NEMATIC LIQUID CRYSTAL DISPLAY WITH HOMEOTROPIC ALIGNMENT ANDNEGATIVE DIELECTRIC ANISOTROPY”, whose patentee is Case Western ReserveUniversity and in “Eurodisplay '93”, pp. 158-159 by Takahashi et al.

On the contrary to the TN mode, the alignment of the liquid crystalmolecules of the VATN mode liquid crystal display in off state issimilar to that of the TN mode in on state, that is, the liquid crystalmolecules align perpendicular to the substrates. In the on state, themolecular axes of the liquid crystal molecules are aligned parallel tothe substrates and twisted spirally by a constant pitch from onesubstrate to the other substrate, and the director of the liquid crystallayer varies continuously.

In case of VATN mode liquid crystal display in normally black mode,sufficient darkness in off state because the molecular axes of theliquid crystal molecules are aligned vertically to the substrate whenthe electric field is applied.

The display characteristics of VATN may become better by optimizing theparameters such as the ratio d/p of the cell gap d to the pitch p of theliquid crystal layer, the difference of refractive indices Δn between inboth directions and the retardation value Δn*d.

In the mean time, because of the refractive anisotropy of the liquidcrystal material, the retardation value Δn*d changes as the viewingdirection, thereby causing the difference of the intensity and thecharacteristics of light. Therefore, TN displays have the change incontrast ratio, color shift, gray inversion, etc. according to theviewing angle.

TN LCDs with compensation film are developed to compensate thedifference of retardation in liquid crystal layer. However, the filmcompensated TN LCDs still have problems such as inharmony of the displaycharacteristics and gray inversion.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide optimizedcell parameters such as d/p, Δn and Δn*d to improve the opticalcharacteristics of LCDs.

It is another object of the present invention to widen the viewing angleof liquid crystal displays.

It is another object of the present invention to increase the contrastratio of liquid crystal displays.

These and other objects, features and advantages are provided, accordingto the present invention, by a liquid crystal display comprising twoparallel spaced substrates and a liquid crystal layer with negativedielectric anisotropy injected between two substrates wherein the ratiod/p, the cell gap d between two substrates to the pitch p of the liquidcrystal layer, may be equal to or less than 0.3.

The liquid crystal layer is made of a chiral nematic liquid crystal or anematic liquid crystal with 0.01-1.0 wt % of chiral dopant.

On two substrates, alignment layers are formed to align the liquidcrystal molecules vertically to the substrates. The alignment layers mayor may not be rubbed.

The refractive anisotropy Δn may be 0/065-0.123, the cell gap d betweentwo substrates may be 3.0-6.0 μm and the retardation value Δn*d may be0.25-0.4.

When the electric field is not applied, the liquid crystal molecules arearranged vertically to the substrates, and when the sufficient electricfield is applied, the liquid crystal molecules are parallel to thesubstrates and twisted by 90° from one substrate to the other.

These and other objects, features and advantages are also provided,according to the present invention, by a liquid crystal displaycomprising a liquid crystal cell having a liquid crystal material with anegative dielectric anisotropy, and a combination of a-plate, c-plate orbiaxial compensation films attached to the outer surface of the liquidcrystal cell.

The slow axis which is the direction having the largest refractive indexof a-plate or biaxial compensation film may be parallel or perpendicularto the transmission axis of adjacent polarizer.

The difference between the summation of the retardation(n_(xa)−n_(za))*d_(a) of the a-plate compensation film, the retardation(n_(xc)−n_(zc))*d_(c) of the c-plate compensation film, the retardation(n_(xb)−n_(zb))*d_(b) of the biaxial compensation film and theretardation of the polarizers, and the retardation of the liquid crystalcell may be equal to or less than 15% of the retardation of the liquidcrystal cell. The retardation (n_(xa)−n_(ya))*d_(a) of the a-platecompensation film or the retardation (n_(xb)−n_(yb))*d_(b) of thebiaxial compensation film may be 0-100 nm. Here, n_(x), n_(y) and n_(z)are the refractive indices of the x, y and z axes respectively when zaxis is the direction perpendicular to the surface of the liquid crystalcell, x axis is in the surface of the liquid crystal cell and having thelargest refractive index of the a-plate or the biaxial compensation filmand y axis is in the surface of the liquid crystal cell andperpendicular to the x axis, and d is the thickness of the liquidcrystal cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams of the alignment of liquidcrystal molecules of a VATN liquid crystal display respectively in blackstate and white state according to an embodiment of the presentinvention.

FIG. 2 is a sectional view of a VATN liquid crystal cell according tothe present invention.

FIG. 3 is a graph showing transmittance T as a function of voltage Vapplied to the liquid crystal cell for various d/p in the Experiment 1.

FIGS. 4A, 4B and 4C show the viewing angle characteristics for variousd/p in the Experiment 1.

FIG. 5 is a graph showing transmittance T as a function of voltage Vapplied to the liquid crystal cell for various d/p in the Experiment 2.

FIGS. 6A and 6B show the viewing angle characteristics for various d/pin the Experiment 2.

FIG. 7 is a graph showing transmittance T as a function of voltage Vapplied to the liquid crystal cell for various d/p in the Experiment 3.

FIGS. 8A, 8B and 8C show the viewing angle characteristics for variousd/p in the Experiment 3.

FIG. 9 is a graph of the viewing angle in the 45° diagonal direction asa function of the retardation value.

FIG. 10 illustrates the relation between the cell gap and the responsetime.

FIGS. 11A-20 show the structures of the liquid crystal display accordingto the first to the sixteenth embodiments respectively.

FIG. 21 is a graph showing the contrast ratios in right, up, left andbottom 60° directions as a function of the retardation value(n_(x)−n_(y))*d for the LCD according to the tenth embodiment of thepresent invention shown in FIG. 14.

FIG. 22 is a graph showing the contrast ratios in right, up, left andbottom 60° directions as a function of the retardation value(n_(xb)−n_(yb))*d_(b) for the LCD according to the tenth embodiment ofthe present invention shown in FIG. 15.

FIG. 23 is a graph showing the contrast ratios in right, up, left andbottom 60° directions as a function of the retardation value(n_(xb)−n_(yb))*d_(b) for the LCD according to the thirteenth embodimentof the present invention shown in FIG. 17.

FIG. 24 shows the viewing angle characteristics of the VATN LCDaccording to the eleventh embodiment.

FIG. 25 shows the viewing angle characteristics of the VATN LCDaccording to the thirteenth embodiment.

FIG. 26 shows the 3-D luminance of the VATN LCD according to theeleventh embodiment.

FIG. 27 shows the 3-D luminance of the VATN LCD according to thethirteenth embodiment.

FIG. 28A-28C show the 8 gray scale performance of the VATN LCD accordingto the thirteenth embodiment.

FIG. 29A-29D show the 8 gray scale performance of the 2-D VATN LCDhaving the structure of the thirteenth embodiment.

FIG. 30 shows the viewing angle characteristics of the 2-D VATN LCDhaving the structure of the tenth embodiment.

FIG. 31 shows the viewing angle characteristics of the VATN LCD havingthe structure of the eleventh embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the present invention are shown. This invention may, however, beembodied in different forms and should not be construed as limited tothe embodiments set forth herein. Rather, these embodiments are providedso that this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity.

FIGS. 1A and 1B are schematic diagrams of the alignment of liquidcrystal molecules of a VATN liquid crystal display respectively in blackstate and white state according to an embodiment of the presentinvention. FIG. 2 shows the structure of a VATN liquid crystal displayaccording to an embodiment of the present invention.

As shown in FIGS. 1A and 1B, two transparent insulating substrates 1 and2 are opposite and spaced apart from each other, and transparentelectrodes 12 and 120 made of a transparent conductive material such asITO (indium tin oxide). Alignment layers 14 and 140 are formed on theinner surfaces of the substrates 1 and 2. Between two substrates 1 and2, a liquid crystal material layer 100 made of a chiral nematic liquidcrystal having negative dielectric anisotropy or a nematic liquidcrystal material doped with chiral dopant of 0.01-0.3 wt % isinterposed. The ratio d/p, the cell gap d between the substrates 1 and 2divided by the pitch p of the liquid crystal layer 100, is preferablyequal to or less than 0.3, and the retardation value Δn*d is preferablyin the range of 0.25-0.4. On the outer surfaces of two substrates 1 and2, a polarizer 13 and an analyzer 130, which polarize the rays incidenton the liquid crystal material layer 100 and the rays out of the liquidcrystal material layer 100 respectively, are attached. The polarizingdirection of the polarizer 13 attached to the lower substrate 1 isperpendicular to that of the analyzer 130 attached to the uppersubstrate 2. Alignment layers 14 and 15 may be rubbed or not.

FIG. 1A shows the off state that the electric field is not applied,where the molecular axes of the liquid crystal molecules 3 in the liquidcrystal layer 100 are aligned perpendicular to the surface of thesubstrates 1 and 2 by the aligning force of the alignment layers 14 and15.

The polarized light by the polarizer 13 passes through the liquidcrystal layer 100 without changing its polarization. Then, the light isblocked by the analyzer 130 to make a black state.

FIG. 1B shows the on state that the sufficient electric field is appliedto the liquid crystal layer by the electrodes 4 and 5, where the liquidcrystal molecules 3 in the liquid crystal layer 100 are twisted spirallyby 90° from the lower substrate 1 to the upper substrate 2 and thedirector of the liquid crystal layer varies continuously. Near the innersurfaces of two substrates 1 and 2, the aligning force of the alignmentlayers 14 and 15 is larger than the force due to the applied electricfield, and thus the liquid crystal molecules stay vertically aligned.

The polarized light by the polarizer 13 passes through the liquidcrystal layer 100, and its polarization is rotated by 90° according tothe variation of the director of the liquid crystal layer. Accordinglythe light passes through the analyzer 130 to make a white state.

Next, the experiments according to the present invention will bedescribed.

FIG. 2 is a sectional view of a VATN liquid crystal cell according tothe present invention.

As shown in FIG. 2, a liquid crystal layer 100 is interposed between twoparallel substrates 1 and 2 spaced apart from each other. Spacers 200are mixed with the liquid crystal layer 100 to sustain the gap betweenthe substrates 1 and 2. On the inner surfaces of the substrates 1 and 2near the edge of the substrates 1 and 2, a sealant 300 is formed toprevent the liquid crystal material from flowing out of the cell.

Experiment 1

In this experiment, JALS204, JALS572 (Japan Synthetic Rubber Co.) orSE-1211 (Nissan Chemical Co.) which are used for homeotropic alignmentlayers were used as an alignment layer. Plastic spacers having 4.5 μmdiameter were included in the sealant 300, and the diameter of spacersspread on the substrate 1 or 2 to maintain the cell gap was 4.5 μm.Liquid crystal material, which is filled in the gap between thesubstrates, was a twisted nematic liquid crystal material with negativedielectric anisotropy and doped with a dopant MLC 6247 (Merck Co.). Therefractive anisotropy Δn of the liquid crystal was 0.085, the dielectricanisotropy Δε was 4.5, the elastic constant K₁₁ was 15.4 pN, K₂₂ was 5.8pN and K₃₃ was 17.4 pN, and the viscosity was 30 mm²/s. The cell gap dvaries from 3 μm to 6 μm, and the pitch p is adjusted by varying theamount of the added dopant.

FIG. 3 is a graph showing transmittance T as a function of voltage Vapplied to the liquid crystal cell for various d/p, and FIGS. 4A, 4B and4C show the viewing angle characteristics for various d/p in theExperiment 1.

As shown in FIG. 3, the transmittance T increases as d/p decreases. Whend/p is 0.5, the transmittance is less than 10%. However, if d/p is 0.1or 0.3, the transmittance has relatively high value more than 18%. Thevoltage difference between the electrodes is in the range of 0-10 V.

As a result, the transmittance T increases as d/p decreases, and thetransmittance is high when the d/p is less than 0.3.

When d/p is 0.5, the viewing angles in up, down, left and rightdirections are about 40° and those in diagonal directions are less than90° partly as shown in FIG. 4C. However, as shown in FIGS. 4A and 4B,when d/p is 0.1 or 0.3, the viewing angles in up, down, left and rightdirections are larger than 40° and those in diagonal directions are morethan 90°.

Experiment 2

In this experiment, the structure of liquid crystal cell was the same asin the first experiment except the alignment layers. In Experiment 2, analignment layer 14 was rubbed in the direction of 45° polar angle. As aresult of the rubbing, the liquid crystal molecules were tilted by about1° with respect to the perpendicular direction to the substrate.

FIG. 5 is a graph showing transmittance T as a function of voltage Vapplied to the liquid crystal cell for various d/p, and FIGS. 6A and 6Bshow the viewing angle characteristics for various d/p in the Experiment2.

As shown in FIG. 5, when d/p is 0.5, the transmittance is less than 10%for the sufficient applied voltage. However, when d/p is 0.1, thetransmittance increases to more than 10% as the voltage increases, andwhen d/p is 0.3, the transmittance is over 20%. The voltage differencebetween the electrodes is in the range of 0-5 V.

FIGS. 6A and 6B show the viewing angle characteristics. When d/p is 0.5,as shown in FIG. 6B, the viewing angles in up and down directions aremore than 90° but those in left and right directions are less than 90°.However, as shown in FIG. 6A, in the case that d/p is 0.3, all theviewing angles in up, down, left and right directions are over 90°.

Experiment 3

In this experiment, the structure of the liquid crystal cell was thesame as in the first experiment except the alignment layers. InExperiment 3, two alignment layers 14 and 15 were rubbed in thedirection of 0° polar angle and 90° polar angle respectively, thereforethe rubbing directions of two alignment layers 14 and 15 areperpendicular to each other.

FIG. 7 is a graph showing transmittance T as a function of voltage Vapplied to the liquid crystal cell for various d/p, and FIGS. 8A, 8B and8C show the viewing angle characteristics for various d/p in theExperiment 3.

As shown in FIGS. 7 and 8A-8C, the transmittance T decreases as d/pincreases, and thus the contrast ratio decreases and the viewing anglecharacteristics become worse.

As shown in FIG. 7, when d/p is 0.5, the transmittance is less than 10%for the sufficient applied voltage. However, when d/p is 0.3, thetransmittance increases to more than 25% as the voltage increases, andwhen d/p is 0.1, the transmittance is over 35%. The voltage differencebetween the electrodes is in the range of 0-5 V.

As shown in FIGS. 8A and 8C, when d/p is 0.1 or 0.5, the viewing anglesin up, down, left and right directions are larger than 90°. However, asshown in FIG. 8B, when d/p is 0.3, the viewing angles in up, down, leftand right directions are about 90°.

Experiment 4

In this experiment, the dependency of the viewing angle characteristicsin diagonal directions on the retardation value Δnd was tested. As inExperiment 3, all the alignment layers 14 and 15 were rubbed. Δn variesin the range of 0.065-0.123 and the cell gap d varies in the range of3.0-6.0 μm to vary the retardation value Δnd in the range of 0.3-0.6.

FIG. 9 is a graph of the viewing angle in the 45° diagonal direction asa function of the retardation value.

As shown in FIG. 9, the retardation value Δnd is preferably equal to orless than 0.4 to get a wide viewing angle over 140°.

Experiment 5

In this experiment, the dependency of the response time on the cell gapand d/p was measured. Two test cells were used having the cell gap of4.0 μm and 4.5 μm respectively, and d/p varied in the range of 0.1-0.5by adjusting the amount of the added dopant. Other conditions were thesame as in Experiment 4.

FIG. 10 is a graph of the response time as a function of the cell gapand d/p.

As shown in FIG. 10, the response time is shorter as the cell gap andd/p is smaller. If the cell gap is narrow, the electric field isstronger than when the cell gap is small, and, therefore, the liquidcrystal molecules respond faster. The response time increases as d/pincreases because the viscosity of the liquid crystal material increasesas the density of dopant increases.

Now, film-compensated VATN liquid crystal displays according to theembodiments of the present invention will be described in detail. FIGS.11A-20 show the structures of VATN liquid crystal displays according tothe embodiments.

A VATN liquid crystal display according to the first embodiment of thepresent invention is shown in FIG. 11A.

A liquid crystal cell 50 having a liquid crystal material with negativedielectric anisotropy such as that as shown in FIG. 1A is prepared. Apolarizer 10 is attached on the outer surface of rear side of the liquidcrystal cell 50. On the outer surface of the front side of the liquidcrystal cell 50 an a-plate compensation film 20, a c-plate compensationfilm 30 and another polarizer 11 are attached in sequence. The buffingdirection of an alignment layer (such as 14 in FIG. 1A) on the innersurface of the front side of the liquid crystal cell 50 up-right. Thesolid arrow indicates the buffing direction of the alignment layer thefront side of the liquid crystal cell. On the contrary, the buffingdirection of an alignment layer (such as 15 in FIG. 1A) on the innersurface of the rear side of the liquid crystal cell 50 is down-right.The dotted arrow indicates the buffing direction of the alignment layeron the rear side of the liquid crystal cell. In the first embodiment ofthe present invention, the liquid crystal cell operates in “e” mode thatthe polarizing directions of the polarizers 10 and 11 represented bybidirectional arrows are parallel to the buffing directions of theneighboring alignment layers.

An a-plate compensation film has the refractive indices n_(x), n_(y) andn_(z) satisfying the relation n_(x)>n_(y)=n_(z), and a c-platecompensation has the refractive indices satisfying the relationn_(x)=n_(y)>n_(z), where n_(x), n_(y) and n_(z) are respectively therefractive indices in the x-direction, y-direction and z-direction whenthe z-direction is perpendicular to the surface of the liquid crystalcell 50, and the x-axis and y-axis spans a plane parallel to the surfaceof the liquid crystal cell 50.

The x-axis of the a-plate compensation film, which has a largestrefractive index (slow axis), may match with or be perpendicular to thepolarizing direction of its neighboring polarizer. If not, the lightleakage may be generated to reduce the contrast ratio. In the case thatthe liquid crystal display shown in FIG. 11A, the x-axis of the a-platecompensation film 20 is in the up-right direction and parallel to thepolarizing direction of the polarizer 11. The solid bidirectional arrowdrawn on the a-plate compensation film 20 indicates the direction of thex-axis of the a-plate compensation film 20.

According to the second embodiment, as shown in FIG. 11B, the positionsof a-plate and c-plate compensation films are exchanged. On the rearside of the liquid crystal cell 50, a polarizer 10 is attached. Ana-plate compensation film 20, a c-plate compensation film 30 and apolarizer 11 is attached on the front side of the liquid crystal cell 50in sequence. The buffing directions of the alignment layer, thepolarizing directions of the polarizer 10 and 11 and the x-axis of thea-plate compensation film 20 are similar to those of the firstembodiment.

Liquid crystal displays according to the third and the fourthembodiments are shown in FIGS. 11C and 11D respectively, where thecompensation films 20 and 30 are inserted between the rear polarizer 10and the liquid crystal cell 50. According to the third embodiment, ac-plate compensation film 30, an a-plate compensation film 20 and apolarizer 10 are attached in sequence on the rear side of the liquidcrystal cell 50 (FIG. 11C). In the fourth embodiment, the positions ofthe c-plate compensation film 30 and the a-plate compensation film 20are opposite to those in the third embodiment (FIG. 11D). The buffingdirections of the alignment layers and the polarizing directions of thepolarizers 10 and 11 are similar to those of the first embodiment. Thex-axis of the a-plate compensation film 20 is matched with thepolarizing direction of the rear polarizer 10 neighboring to the a-platecompensation film 20.

FIGS. 12A-12D illustrate liquid crystal displays according to the fifthto the eighth embodiment, and the liquid crystal displays operate in “o”mode. Arrangements of a liquid crystal cell 50, polarizers 10 and 11, ana-plate compensation film 20 and a c-plate compensation film 30 aresubstantially the same as those of the liquid crystal displays shown inFIGS. 11A-11D. The difference is that the liquid crystal displaysoperate in “o” mode, that is, the buffing directions of alignment layersare perpendicular to the polarizing directions of the neighboringpolarizers, and that the buffing directions of the alignment layers areperpendicular to x-axis of the a-plate compensation films.

FIG. 13 illustrates a liquid crystal display according to the ninthembodiment. The liquid crystal display has another c-plate compensationfilm 31 inserted between the liquid crystal cell 50 and the rearpolarizer 10 in the first embodiment. The liquid crystal displayaccording to the tenth embodiment, as shown in FIG. 14, has anothera-plate compensation film 21 inserted between the c-plate compensationfilm 31 and the rear polarizer 10 in the ninth embodiment, and thex-axis of the a-plate film 21 is parallel to the polarizing direction ofthe rear polarizer 10.

A liquid crystal display according to the eleventh embodiment is shownin FIG. 15. A rear polarizer 10 is attached to the rear side of a liquidcrystal cell 50 having a liquid crystal material with negativedielectric anisotropy. On the opposite side of the liquid crystal cell,a biaxial compensation film 40 is interposed between the liquid crystalcell 50 and a front polarizer 11. The biaxial compensation film has therefractive indices satisfying the relation that n_(x)>n_(y)>n_(z). As inthe embodiments described above, the x-axis of the biaxial compensationfilm may be parallel or perpendicular to the polarizing direction of itsneighboring polarizer. It may operate in “e” mode or “o” mode.

According to the twelfth embodiment, a biaxial compensation film 40 isinterposed between a rear polarizer 10 and the rear side of a liquidcrystal cell 50, and a front polarizer 11 is attached directly on thefront side of the liquid crystal cell 50, as shown in FIG. 16. As in theembodiments described above, the x-axis of the biaxial compensation filmmay be parallel or perpendicular to the polarizing direction of itsneighboring polarizer. It may operate in “e” mode or “o” mode.

As shown in FIG. 17, according to the thirteenth embodiment, two biaxialcompensation films 40 and 41 are inserted between respective polarizers10 and 11 and the front and rear side of a liquid crystal cell 50,respectively. As in the embodiments described above, the x-axis of thebiaxial compensation film may be parallel or perpendicular to thepolarizing direction of its neighboring polarizer. It may operate in “e”mode or “o” mode.

A liquid crystal display according to the fourteenth embodiment is shownin FIG. 18. A rear polarizer 10 is attached to the rear side of a liquidcrystal cell 50 having a liquid crystal material with negativedielectric anisotropy. On the opposite side of the liquid crystal cell,a biaxial compensation film 40 and a c-plate compensation film 30 areinserted between the liquid crystal cell 50 and a front polarizer 11.The x-axis of the biaxial compensation film 40 is parallel to thepolarizing direction of its neighboring polarizer 11. The liquid crystalcell operates in “e” mode that the polarizing directions are parallel tothe buffing directions of the neighboring alignment layers in thisembodiment. The position of the biaxial compensation film 40 and thec-plate compensation film 30 may be exchanged, and they may be insertedbetween a rear polarizer 10 and the liquid crystal cell 50.

Another c-plate compensation film 31 may be added between the rearpolarizer 10 and the liquid crystal cell of the fourteenth embodiment,according the fifteenth embodiment as shown in FIG. 19. A c-platecompensation film and a biaxial compensation film may be added to theLCD of fourteenth embodiment as in the sixteenth embodiment (FIG. 20).The x-axis of the biaxial compensation film may be perpendicular to thepolarizing direction of the neighboring polarizer, and the liquidcrystal display may operate in “o” mode.

In the meantime, the contrast ratio CR for the VATN LCD is defined asfollows:

CR=(luminance)_(ON)/(luminance)_(OFF).

That is, the contrast ratio in normally black mode is the value that(luminance)_(ON), which is the luminance at the state that the voltageis applied (on state), divided by (luminance)_(OFF), which is theluminance at the state that the voltage is not applied (off state). Thecontrast ratio may be drastically improved if somehow the luminance inoff state is further reduced.

The viewing angle characteristics and gray scale performance of the filmcompensated VATN LCD according to the present invention were calculatedby optical simulation program. The geometrical structure and relevantparameters used in the simulation are summarized in Table. 1. It wasassumed that the polarizer itself gives retardation of −60 nm.

TABLE 1 The parameters of VATN cells for optical simulations Elasticconstant (pN) K₁ 16.6 K₂ 6.5 K₃ 18.5 Relative dielectric ε_(ll) 3.5constant ε⊥ 7.7 Refractive index n_(e) 1.5584 n_(o) 1.4757 Pretilt angleθ_(p) 89° Twist angle Φ_(TN) 90° Cell gap (μm) d 4.0 Cell gap divided byPitch d/p 0.1 Off state voltage (V) V_(off) 0 On state voltage (V)V_(on) 5 Buffing direction front  45° or 225° back 315° or 135°

The net retardation value due to all retardation films (including thepolarizers), (n_(x)−n_(z))*d, is preferably equal to that of the liquidcrystal cell birefringence. In our case, the liquid crystal cellbirefringence was around 320 nm. However, the biaxiality of thecompensation films may be optimized in the individual case.

FIG. 21 is a graph showing the contrast ratios in right, up, left andbottom 60° directions as a function of the retardation value(n_(x)−n_(y))*d for the LCD according to the tenth embodiment of thepresent invention shown in FIG. 14.

Referring to FIG. 14, the structure of the VATN LCD according to thetenth embodiment of the present invention is more fully described. Aliquid crystal cell 50 has a liquid crystal material with negativedielectric anisotropy. On the front side of the liquid crystal cell 50,a c-plate compensation film 30 and an a-plate compensation film 20 isattached in sequence, and a polarizer 11 is attached thereto. On therear side of the liquid crystal cell 50, a c-plate compensation film 31and an a-plate compensation film 11 is attached in sequence, and apolarizer 11 is also attached thereto. The polarizing directions of thepolarizers 10 and 11 are parallel to the buffing direction of theirneighboring alignment layers of the liquid crystal cell 50. That is, theliquid crystal cell operates in “e” mode. The orientation of x-axis(slow axis) of a-plate compensation film 20 and 21 matches with thepolarizing direction of its neighboring polarizer.

The optimum retardation value (n_(xa)−n_(ya))*d_(a) of the a-platecompensation film is around 20 nm as shown in graph of FIG. 21 wheren_(xa) and n_(ya) are respectively the refractive indices along thex-axis and the y-axis of the a-plate compensation films, and d_(a) isthe thickness of the a-plate compensation film. The maximum contrastratio is 140:1 in up 60° direction.

FIG. 22 is a graph showing the contrast ratios in right, up, left andbottom 60° directions as a function of the retardation value(n_(xb)−n_(yb))*d_(b) for the LCD according to the tenth embodiment ofthe present invention shown in FIG. 15 where n_(xb) and n_(yb) arerespectively the refractive indices along the x-axis and the y-axis ofthe biaxial compensation films, and d_(b) is the thickness of thebiaxial compensation film. The optimum retardation value(n_(xb)−n_(yb))*d_(b) of the biaxial compensation film is around 50 nmas shown in FIG. 22.

If a biaxial compensation film is used as in the eleventh embodiment ofthe present invention, the contrast ratio may be more improved. As shownin FIG. 22, the contrast ratio is over 175:1 in up 60° direction. Thecontrast ratio in left and right 60° directions are about 250:1 and175:1, respectively.

FIG. 23 is a graph showing the contrast ratios in right, up, left andbottom 60° directions as a function of the retardation value(n_(xb)−n_(yb))*d_(b) for the LCD according to the thirteenth embodimentof the present invention shown in FIG. 17.

Referring to FIG. 17, the structure of the VATN LCD according to thethirteenth embodiment of the present invention is described in detail. Aliquid crystal cell 50 has a liquid crystal material with negativedielectric anisotropy. Both on the front and rear side of the liquidcrystal cell 50, two biaxial compensation films 40 and 41 and twopolarizers 11 and 10 are attached respectively. The liquid crystal celloperates in “e” mode that the polarizing directions of the polarizers 10and 11 are parallel to the buffing directions of their neighboringalignment layers of the liquid crystal cell 50. The orientation ofx-axis (slow axis) of biaxial compensation film 40 and 41 matches withthe polarizing direction of its neighboring polarizer.

The optimum retardation value (n_(xb)−n_(yb))*d_(b) of the biaxialcompensation film is around 40 nm as shown in FIG. 23. The contrastratio is as good as in the eleventh embodiment of the present invention.

The contrast ratio in up 60° direction is about 200:1, and the contrastratios in left and right 60° directions are over 250:1.

In the tenth embodiment of the present invention, if the retardationvalue (n_(xc)−n_(zc))*d_(c) of the c-plate compensation films 30 and 31is 100 nm, the net retardation value of all retardation films (includingthe polarizers), (n_(x)−n_(z))*d, is 360 nm because the retardationvalue of the a-plate compensation film is 20 nm and the retardationvalue of the polarizer is 60 nm. This value is nearly equal to theliquid crystal cell birefringence.

In the eleventh embodiment of the present invention, if the retardationvalue (n_(xb)−n_(zb))*d_(b) of the biaxial compensation films 40 is 200nm, the total retardation value of all retardation films,(n_(x)−n_(z))*d, is 320 nm. This value is equal to the liquid crystalcell birefringence.

In the thirteenth embodiment of the present invention, if theretardation value (n_(xb)−n_(zb))*d_(b) of the biaxial compensationfilms 40 and 41 is 100 nm, the net retardation value of all retardation,(n_(x)−n_(z))*d, is 320 nm as in the eleventh embodiment of the presentinvention.

As shown in the above, if the difference between the net retardationvalue of all retardation, (n_(x)−n_(z))*d, and the liquid crystal cellbirefringence is relatively small, the viewing characteristics are muchimproved. The difference is preferably equal to or less than 15% of theliquid crystal cell birefringence.

FIGS. 24 and 25 show the viewing angle characteristics of the filmcompensated VATN LCDs according to the eleventh and thirteenthembodiments of the present invention, respectively. The viewing anglecharacteristics are dramatically improved compared with the cases thatthe compensation film is not used and that c-plate compensation filmsare used.

FIGS. 26 and 27 show the 3-dimensional luminance distributions of thefilm compensated VATN LCDs according to the eleventh and thirteenthembodiments of the present invention, respectively. A VATN LCD in offstate has the intrinsic light leakage in 45° direction relative to thepolarizing direction. The c-plate compensation film with optimum valuemay reduce the light leakage by 10 times. The optimum black states ofthe VATN LCDs may be achieved by using the biaxial compensation filmsaccording to embodiments of the present invention. It results in theextremely high contrast ratio.

The eight gray scale performance of the VATN LCD according to thethirteenth embodiment using the optimum biaxial compensation film iscalculated, and the calculated result is shown in FIGS. 28A-28C. FIG.28A is the graph of the luminance as a function of the azimuthal anglein the plane making 45° relative to the polarizing directions. FIG. 28Bis the graph of the luminance as a function of the azimuthal angle inthe vertical plane to the polarizing direction, and FIG. 28C is thegraph of the luminance as a function of the azimuthal angle in thehorizontal plane to the polarizing direction. Although the contrast ofthe display gets improved a lot in theses compensation modes, the grayscale performances are not so good enough.

If the compensation configurations according to the embodiments of thepresent invention are adapted to 2-domain VATN LCD, the gray scaleperformance gets greatly improved. FIGS. 29A-29D show the gray scaleperformance of 2-D VATN LCD having the compensation configurationaccording to the thirteenth embodiment of the present invention. FIG.29A is the graph of the luminance as a function of the azimuthal anglein the horizontal plane to the polarizing direction. FIG. 29B is thegraph of the luminance as a function of the azimuthal angle in thevertical plane to the polarizing direction, and FIGS. 29C and 29D arerespectively the graph of the luminance as a function of the azimuthalangle in the planes making 45° and 135° relative to the polarizingdirection.

FIGS. 30 and 31 show the gray scale performances of 2-D VATN LCDs havingthe compensation configurations according to the tenth and the eleventhembodiments of the present invention, respectively.

All compensation configurations shown in FIGS. 11A-20 can be used for2-D or multi-domain VATN LCDs as long as the buffing directions matchwith the x-axis (slow axis) of the biaxial or the a-plate compensationfilm.

Moreover, the film compensated VATN LCDs according to the presentinvention may be adapted to ECB (electrically controlled birefringence)VATN LCDs, fringe controlled multi-domain VATN LCDs, IPS (in-planeswitching) mode VATN LCDs, etc.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

What is claimed is:
 1. A liquid crystal display comprising: two spacedapart substrates, each substrate having an electrode; and a liquidcrystal layer with negative dielectric anisotropy which is interposedbetween the two substrates and homeotropically aligned; wherein cell gapbetween the substrates divided by pitch of the liquid crystal layer isequal to or less than 0.3.